In various medical fields, the use of artificial stimulation devices, or prosthesis, to stimulate damaged cells and/or tissue which are no longer responsive to natural stimuli is well known. These devices mimic natural impulses and act to re-establish the natural stimulation path.
One of the best examples of the success of such an approach is the use of the cochlear implant to restore partial hearing in profoundly deaf people. A person is diagnosed as profoundly deaf if either a very large number of hair cells or auditory neurons throughout the cochlea, the spiral-shaped cavity of the inner ear, are damaged. Cochlear implants use electrical stimulation to directly excite the remaining auditory neurons which connect the ear to the brain. In general, such implants include a microphone which picks up sound, an array of electrodes surgically inserted into the cochlea, which electrically stimulates functional auditory neurons of the cochlea, and a signal transmission system which transmits the sound information from the microphone to the array of electrodes. The whole system is designed so that activation of the electrodes will fire up the neurons, which communicate with the patient's central nervous system, and thereby transmit information about the acoustic signal to the brain.
In practice, implementation of existing cochlear implant technology is impeded by the size of the wires used to transmit information to the neurons. The minimum diameter of such a wire being about 25 μm (P. Åke Öberg, Tatsuo Togawa, Francis A. Spelman (eds.), Sensors in Medicine and Health Care, Sensors Applications Volume 3, Wiley-VCH Verlag GmbH & Co. KGaA, 2004), the number of wires is limited to less than one hundred (100) by the diameter of the auditory canal. By increasing the number of electrodes, it is hoped that the resolution of the perceived acoustic signal can be improved. Moreover, by decreasing the diameter of the wire, the risk of injury to the cochlea and its inner structure, which includes the basilar membrane and the hair cells, is reduced. This risk of injury inherent with electrical charge is of import given the increase in popularity of cochlear implants and their growing consideration for use in patients with residual hearing. One other solution would be to develop a device which uses non-electrical artificial stimulation, for example optical or photo-stimulation. US Patent Application No. 2005/0216072 (MAHADEVAN-JANSEN) discloses a system and methods for optical stimulation of neural tissues. However, one major drawback with this system and these methods lies in the probe: the probe delivers optical energy to the target neural tissue, one site at a time and at a distance away from the target neural tissue.
Applications of electrical stimulation systems are not limited to cochlear implants. They include brain neuro-stimulation (pain relief, tremor control, treatment of cerebral palsy, treatment of Parkinson's disease, visual cortex implants for the blind), spinal neuro-stimulation (pain relief, peripheral vascular flow enhancement), peripheral nerve stimulation (pain relief, phrenic nerve pacing), retinal implants, heart pacemakers, tissue-growth stimulation and inhibition, etc.
Functional Electrical Stimulation (FES) is used to produce, by means of electrical stimulation, contractions in muscles either injured or paralysed due to central nervous system lesions. In the case of FES, arrays of electrodes are implanted under the skin and used to choreograph movement in the patient's muscles.
Applications for this approach are found, for example, in cases of stroke, spinal cord injury, head injury, cerebral palsy, and multiple sclerosis. Here, too, resolution is limited by the size of the wires used for electrical stimulation.
Efforts are underway to develop visual prostheses, both retinal and cortical. Retinal prostheses aim to restore some form of vision to patients that are blind owing to a degenerative condition, such as retinitis pigmentosa or age-related macular degeneration, by bypassing the photoreceptor cells of the retina which have become dysfunctional and electrically stimulating the relatively intact retinal ganglion cells which connect the eye to the visual cortex of the brain. Electrical stimulation of the retinal ganglion cells creates the sensation of a spot of light (or phosphene) in the spatial vicinity of the stimulation. Cortical prostheses may be used to treat patients with secondary blindness not due to retinal or optic nerve disease. The difficulty with cortical implants lies in the need for intracranial surgery and the complexity of brain geometry. Nevertheless, both types of prostheses are faced with the problems inherent with electrical stimulation: injury incurred by neurons under chronic use and lack of specificity. U.S. Pat. No. 6,458,157 (SUANING) discloses an apparatus in which all tissue-contacting components may be fabricated from materials known to be well tolerated by human tissue. While SUANING discloses attempts that have been made to limit injury due to long-term use, the matter of specificity is not expressly addressed.
In general, traditional methods and devices for direct electrical neuro-stimulation lack spatial, physiological and strength specificities. Furthermore, they are prone to electrical interference from the environment. For example, electrical stimulation of the visual cortex produces phosphenes (or blurred) spots rather than pixel-like (or well-defined) spots. Stimulating tactile sense through electrical stimulation of specific neuronal cells is practically impossible without stimulating muscles and/or a temperature response, producing hitching or pain. A stimulation device permitting stimulation of specific neural ganglion cells would allow for better control of the stimulation process.
While certain cell, tissue, or system functions can be affected or controlled through electrical stimulation, a more efficient means of regulating these functions would be through the use of natural biochemical stimulators or inhibitors that are target specific. For example, insulin is produced naturally by the pancreas and is used by the body to activate glucose metabolism. Insulin production cannot be induced through electrical stimulation. Diabetics, who count for more than 5% of North Americans, must inject themselves with insulin in order to metabolise the glucose present in their body. A more convenient means of regulating the level and production of insulin would greatly benefit diabetics. The same holds true for people that must take medications regularly either orally or through injection.
Recent developments in nanotechnology (nanoshells, quantum dots (QDs), micelles), photodynamic therapy and photo-imaging offer new possibilities for improving specificity. These new technologies provide ways to cage, tag and locate molecules thus allowing the regulation and monitoring of optical stimulation mechanisms. Of particular interest are molecular structures or compounds that undergo changes in their properties (chemical affinity, conformal structure or composition) upon exposure to light (photoactivated changes). Following photoactivation, these molecules can react with other molecules or cells or emit light. In some cases, molecules undergo photoactivation only in the presence of certain other molecules or cells thus allowing these photoactivated molecules to be used as targets for locating, monitoring, imaging or destroying these other molecules or cells when lighted. For example, U.S. Pat. No. 6,668,190 (IEZZI et al.) discloses a drug delivery system that includes a fluid channel for delivering a drug to one of a number of sites and a light channel for delivering light to an area near one of the sites for photoactivating caged and/or non-caged molecules of the drug to stimulate neurological tissue.
From all of the above, there is a need for an improved manner of delivering either electrical or optical stimulations to specific stimulation sites of any type.